1. Field of the Invention
The present invention generally relates to memory circuits and systems, and more particularly, the present invention relates to devices and methods for controlling active termination resistors which are used to improve signaling characteristics in memory circuits and systems.
2. Description of the Related Art
Generally, as the bus frequency of a memory system (e.g., a memory system employing DRAM devices) increases, the signal integrity within the memory system degrades. Thus, a variety of bus topologies capable of reducing signal distortion have been developed. For example, it is known that the use of resistive terminations at either the receiver and/or transmitter sides within the memory system is an effective means for absorbing reflections and thereby improving signal performance. Resistive termination configurations of this type generally fall into one of two categories, i.e., passive termination or active termination.
FIG. 1 shows an example of a passive resistive termination in a memory system. In particular, a so-called stub series terminated logic (SSTL) standard is illustrated in which the bus of a memory system 100 is connected to termination voltages Vterm through termination resistors Rterm, and DRAM-mounted memory modules are inserted into slots having predetermined stub resistors Rstub. In this case, the stub resistors Rstub are not mounted on the DRAM chips, and accordingly, the example here is one of an “off-chip” passive resistive termination.
When used in a double data rate (DDR) memory system, the passive resistive termination of the SSTL standard is capable of ensuring a data rate of about 300 Mbps. However, any increase in data rate beyond 300 Mbps tends to degrade signal integrity by increasing the load of the bus having the resistive stubs. In fact, a data rate of 400 Mbps or greater is generally not achievable with the SSTL bus configuration.
FIG. 2 shows an example of a memory system having an active resistive termination, and in particular, an active-termination stub bus configuration. Here, each chipset for controlling the operation of the memory modules, and DRAMs mounted on the respective modules, includes an active termination resistor Rterm. The active termination resistor Rterm is mounted “on-chip” and may be implemented by complementary metal oxide semiconductor (CMOS) devices. In this memory system, active bus termination is achieved through input/output (I/O) ports mounted on the modules.
Each combination of one or more resistive elements Rterm and one or more ON/OFF switching devices in each DRAM is generally referred to herein as an “active terminator”. Active terminators can take on any number of different configurations. FIG. 3 illustrates an example of an active terminator having a center-tapped termination which is described in U.S. Pat. No. 4,748,426. In this example, the effective active termination resistance Rterm of the circuit can be varied between different values (e.g., 150 ohms and 75 ohms) depending on the enable/disable state of signals ON/OFF_1 and ON/OFF_2.
When a DRAM mounted in a memory module is not accessed (e.g., not read or written), the active termination resistor Rterm thereof is enabled by connecting the same to the bus to improve signal integrity. In contrast, when a DRAM is accessed (e.g., read or written), the active termination resistor Rterm thereof is disabled and disconnected from the bus to reduce load.
However, a considerable amount of time is required to enable the active termination resistors installed in the DRAM circuits in response to the active termination control signals, and when a module-interleaved write/read operation is performed, this time lapse can result in data bubbles, thereby degrading memory system performance.
DRAMs which include a delay locked loop (DLL) or phase locked loop (PLL) can overcome this problem by controlling the enabling/disabling of the active termination resistor thereof in synchronization with an external clock. However, in the case where the DLL or PLL is deactivated during a power down or standby mode of DRAMs of a corresponding memory module, enabling/disabling of the active termination resistor cannot be controlled.